Compact optical fiber photoacoustic gas sensor with integrated multi-pass cell

High-Precision Multipass Fiber-Optic Photoacoustic Gas Analyzer Based on 2f/1f Wavelength Modulation Spectroscopy

A self-calibration fiber-optic photoacoustic (PA) gas analyzer based on 2f/1f wavelength modulation spectroscopy (WMS) is proposed, which utilizes gas and solid multipass absorption enhancement. The laser light is incident obliquely on the cell wall, and one end of the cell is equipped with a highly reflective mirror. The gas analyzer takes full advantage of the miniature multipass PA cell, which enhances the absorption of gas and solid simultaneously. As a result, the double absorption enhancement of 1f and 2f PA signals are realized. A dual-channel lock-in white-light interferometer based on fiber-optic PA demodulation is designed to simultaneously extract the 1f and 2f PA signals detected by the silicon cantilever. The experimental results of methane gas detection show that the minimum detection limit (MDL) of the PA gas analyzer is 20 ppb when the integration time is 60 s. Moreover, the detection error of gas concentration is within 3% when the laser power is reduced by half. The fiber-optic PA gas analyzer eliminates the influence of changes in the laser power and optical path loss, which can be used for the high-precision detection of trace gases.

Integrated near-infrared fiber-optic photoacoustic sensing demodulator for ultra-high sensitivity gas detection

An integrated near-infrared fiber-optic photoacoustic sensing demodulator was established for ultra-high sensitivity gas detection. The demodulator has capacities of interference spectrum acquisition and calculation, laser modulation control as well as digital lock-in amplification. FPGA was utilized to realize all the control and signal processing functions, which immensely improved the integration and stability of the system. The photoacoustic signal detection based on fiber-optic Fabry-Perot (F-P) acoustic sensor was realized by applying ultra-high resolution spectral demodulation technique. The detectable frequency of photoacoustic signal achieved 10 kHz. The system integrated lock-in amplification technology, which made the noise sound pressure and dynamic response range of sound pressure detection reached 3.7 μPa/√Hz @1 kHz and 142 dB, respectively. The trace C2H2 gas was tested with a multi-pass resonant photoacoustic cell. Ultra-high sensitivity gas detection was accomplished, which was based on high acoustic detection sensitivity and the matching digital lock-in amplification. The system detection limit and normalized noise equivalent absorption (NNEA) coefficient were reached 3.5 ppb and 6.7 × 10-10 cm-1WHz-1/2, respectively. The devised demodulator can be applied for long-distance gas measurement, which depends on the fact that both the near-infrared photoacoustic excitation light and the probe light employ optical fiber as transmission medium.

Properties of a Symmetrical Photoacoustic Helmholtz Cell Operating with Imbalanced Counterphase Light Stimulation

The output signal from a photoacoustic cell based on a symmetrical Helmholtz resonator structure can be substantially increased if a counterphase light stimulation is applied to the cell cavities. However even slight differences in the intensity of the light beams irradiating the cavities may affect the frequency response of the cell and the output signal level. This paper shows the influence of the imbalanced light irradiation on the properties of such a cell. It was found that even at relatively high irradiation mismatch, and even with the photoacoustic signal detection implemented with a single microphone, the influence of the irradiation imbalance on the frequency response of the cell around the resonance frequency is not critical. In the case of differential detection of the photoacoustic signal, the imbalance of the light irradiation does not affect the frequency response of the cell, but only the output signal level.